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Posted: March 27, 2008

Anticancer siRNA therapy advances, thanks to nanoparticles

(Nanowerk News) Small pieces of nucleic acid, known as siRNAs (short interfering RNAs), can turn off the production of specific proteins, a property that makes them one of the more promising new classes of anticancer drugs in development. Indeed, at least two siRNA-based anticancer therapies, both delivered to tumors in nanoparticles, have begun human clinical trials. Now, three new reports highlight the progress that researchers are making in developing broadly applicable, nanoparticle-enabled siRNA anticancer therapeutics.

In the first report, Mark E. Davis, Ph.D., an investigator in the Nanosystems Biology Cancer Center at the California Institute of Technology, and former graduate student Derek Bartlett, Ph.D., now at the City of Hope, used mathematical modeling and results from dosing experiments in a mouse model of human cancer to explain therapeutic response with various dosing regimes for both targeted and untargeted siRNA-containing nanoparticles. The results of this work, published in the journal Biotechnology and Bioengineering, provide guidelines for optimizing the design of siRNA-based anticancer therapies ("Impact of tumor-specific targeting and dosing schedule on tumor growth inhibition after intravenous administration of siRNA-containing nanoparticles").

In their experiments, the investigators used a cyclodextrin-based nanoparticle to deliver an siRNA agent designed to reduce production of ribonucleotide reductase subunit M2 (RRM2), which plays an important role in tumor growth. The investigators created two versions of their nanoparticle formulation, one targeted to transferrin, a protein overexpressed by many tumors, and the other untargeted. They also used two different dosing regimens, one consisting of three consecutive daily injections, the other consisting of three injections spaced 3 days apart.

Data from these experiments showed that targeted nanoparticles were far more effective than untargeted nanoparticles at reducing tumor growth. Dosing regimen, however, had no statistically significant impact on the outcome for either nanoparticle formulation. Closer examination of tumors removed from the animals following treatment showed that the targeted nanoparticles were able to deliver siRNA into the tumors, although the final distribution of siRNA throughout the tumors was not uniform. The investigators then modeled the observed responses; the results of these simulations led them to conclude that it is not necessary to persistently shut down protein production in order to achieve a therapeutic response using siRNA. Instead, they concluded, it is more important to maximize the number of cells reached with a sufficient dose of siRNA agent.

In a second report, Leaf Huang, Ph.D., and his colleagues at The University of North Carolina at Chapel Hill, describe their development of a self-assembling siRNA-liposomal formulation that they can then coat with poly(ethylene glycol) (PEG) linked to a targeting agent. This targeted liposome was fourfold more effective than an untargeted, but otherwise identical, liposome at delivering siRNA into tumors. Gene silencing activity was also higher for the targeted version, with the therapeutic effect lasting 4 days. The investigators also found that although the targeted nanoparticle effectively penetrated lung metastases, it did not enter liver cells. In addition, the targeted nanoparticle showed little immunotoxicity. These results appear in the Journal of Controlled Release ("Efficient gene silencing in metastatic tumor by siRNA formulated in surface-modified nanoparticles").

Another paper published in the same journal, this one from Stefaan De Smedt, Ph.D., and his collaborators at Ghent University in Belgium, describes a method that could prove useful in both preclinical and clinical studies of nanoparticle-enabled siRNA therapeutics ("A fast and sensitive method for measuring the integrity of siRNA-carrier complexes in full human serum"). Their new technique uses fluorescence fluctuation spectroscopy to measure the stability of these formulations, even at low concentrations, in human serum in less than 1 minute. Serum stability of siRNA-containing nanoparticles is essential to therapeutic efficacy, given that most studies have shown that naked siRNA has little effect on tumors. Using this method, the investigators were able to show that even PEGylated siRNA-containing liposomes were releasing the bulk of their cargo in serum.